The present invention relates generally to uniflow engines and, more particularly, to arrangements for scavenging of such engines.
Two-stroke engines are often categorized by the method by which they achieve gas exchange, i.e., the process of expelling burned gases from a cylinder after combustion and of refilling the cylinder with a fresh charge, e.g. fresh air or a mixture of fresh air and, e.g., fuel. In the field of two-stroke engines, this is called scavenging. Known scavenging designs include cross-, loop-, and uniflow scavenging. Unlike in four-stroke engines, the entire two-stroke scavenging process occurs simultaneously when the piston or pistons are at or near their outermost (bottom dead center) position, and is driven by some external pumping device and not by the motion of the pistons between bottom dead center and top dead center. The filling of a two-stroke cylinder depends on the pressure difference between intake and exhaust ports (valves), how efficiently the in-rushing fresh charge is able to displace the burnt gases from the cylinder without itself exiting the cylinder through the exhaust valves or ports, and how much mass of (mostly) fresh air can be packed into the cylinder by the time that both exhaust and intake ports or valves are closed so that the chamber is sealed.
Regardless of whether the engine is an opposed piston engine or a single piston engine, when the intake is at one end of the cylinder and the exhaust is at the other end, the cylinder and the engine are referred to as having a “uniflow” design or “uniflow scavenged” design. An opposed piston two stroke engine is described herein for purposes of discussion. An opposed piston two stroke engine is a special form of internal combustion engine that includes one, or more cylinder units, each made up of an open cylinder containing two moving pistons, which close off either end of the cylinder, and form a combustion chamber volume between them. Both pistons move in a fixed motion relative to each other and the cylinder so as to create a varying volume between them. This volume forms a combustion chamber. The piston motion is controlled by an external mechanism, most often a slider-crank mechanism, with either two separate cranks held in relative motion by gears or other means, or sharing a single crank. Less commonly, other types of mechanisms, such as a “Scotch yoke” mechanism, are used but the essential operating details here are unchanged. The mechanisms combine the work done by each piston, and convert the linear motion of the pistons to rotational motion, which is the output of the engine. Illustrative structure and operation of opposed piston engines is shown in, for example, U.S. Patent App. Pub. US2013/0036999 which is incorporated by reference.
The innermost position of each piston is referred to as “top center” or “top dead center”, and the outermost position is referred to as “bottom center” or “bottom dead center”, using slider-crank terminology, regardless of the actual mechanism employed, or the physical orientation of the device. A minimum volume occurs when both pistons are simultaneously at or near their top dead center positions, and a maximum volume occurs then both pistons are simultaneously at or near their bottom dead center positions, if the two pistons are configured so that each reaches top dead center and bottom dead center at the same time, then the minimum and maximum volumes coincide with top dead center and bottom dead center, and the two pistons are said to be “in phase”. In the usual case when the two pistons do not achieve top dead center (and bottom dead center) at the same time, then the minimum and maximum volumes occur at approximately the average of the top dead center and bottom dead center, respectively, of each piston, and the pistons are said to be “phased” or “offset” relative to each other.
For a two-stroke engine cycle, the complete cycle, including intake, compression, combustion and exhaust, is completed in one complete motion of the piston from bottom dead center to top dead center and back to bottom dead center, corresponding to one crankshaft revolution. This cycle can be applied to either a positive ignition (spark ignition, or Otto) or a combustion ignition (Diesel) combustion process. The gas exchange process, called “scavenging” in a two-stroke engine, includes expelling (exhausting) the burned gases and relining the cylinder with fresh air (or mixture, if fuel is premixed with the an before entering the cylinder) more or less simultaneously, occurs near bottom dead center, and reduces some of the working stroke of the engine.
The effectiveness of the scavenging process is a critical factor in determining the output of the engine. Usually, either the intake, exhaust or both are through ports (openings in the cylinder wall) near bottom dead center, which are “opened” or “closed” by the piston. While ports are advantageous in allowing a larger flow area than can be accomplished with poppet valves, they have the disadvantage that opening and closing times result from the motion of the piston, and are symmetric about the piston bottom dead center. With an opposed piston engine, both intake and exhaust are through ports, located at opposite ends of the cylinder at maximum volume, each controlled by one of the pistons. This location inherently achieves “uniflow-scavanging”, which provides an advantage for optimal scavenging by separating the intake and exhaust as much as possible, thereby reducing mixing of the fresh and burnt gases, but the use of pistons to control both ports creates a difficulty in timing the opening and closing of the ports to also achieve good scavenging.
The inventor has recognized that optimal port for valve) timing requires two conditions: 1) The exhaust should open before the intake, to allow a “blowdown” of the residual pressure in the cylinder to exhaust, so that the cylinder press is approximately the same as, or below the intake manifold pressure at the time of intake opening and 2) the exhaust should close before the intake to allow a build-up of pressure, and therefore more mass, of fresh air in the cylinder above the exhaust manifold pressure (approximately atmospheric).
These two conditions are difficult to achieve in a two-stroke cycle, where a single piston may control both ports. One solution is the single piston “uniflow” design, which uses piston-controlled ports for intake (usually) and poppet valves for exhaust. However, this design requires a valve train system very similar to that of a four-stroke engine, which reduces the potential cost advantage of a two-stroke engine, and the achievable flow area of the poppet valves may restrict the exhaust flow. With an opposed piston engine, both port opening conditions are usually met by “phasing” or “off-setting” the motion of the two pistons relative to each other, retarding the intake relative to the exhaust. This characteristic allows large port areas for both intake and exhaust to allow high gas flow, which is one of the main advantages of the opposed piston design, and is part of the reason for the historically high output of opposed piston engines relative to other engine designs.
When the two pistons of an opposed piston engine are phased relative tee each other, the intake and exhaust processes can be timed for effective scavenging. However, the motion of neither piston is timed relative to the pressure rise in the cylinder from combustion to achieve the same conversion efficiency of the thermodynamic work of the combustion gas into mechanical work of each piston that can be achieved by conventional single piston engines. In most type of piston engines, the highest conversion of work occurs when combustion is tuned so that the maximum pressure occurs at approximately 10-15 degrees after piston top dead center. The reason for this is that a conventional slider-crank mechanism is “locked” at top dead center, and achieves maximum torque when the mechanism is around mid-stroke, and is again “locked” at bottom dead center. When the two pistons of an opposed piston engine are phased, the best combustion timing will be somewhat late for the leading piston, but will be too early for the trailing piston, with a large amount of the pressure rise trying to push that piston in the reverse direction. This results in high torsional vibration, and also significant periods of “negative torque” of the trailing crankshaft during the cycle, which subtracts from the positive torque of the leading crankshaft, resulting in lower than expected engine output.
It is desirable to reduce the efficiency losses due to sub-optimal alignment of piston motion with the combustion pressure rise with an opposed piston engine. It is also desirable to provide a different solution to port timing of opposed piston and single piston uniflow engines in order to achieve the same scavenging performance.
According to an aspect of the present invention, a uniflow engine comprises a cylinder wall, at least one intake port extending between the cylinder wall and the intake air gallery wall and an intake valve outside of the cylinder and configured to open and close flow communication between the cylinder and the intake air gallery through the at least one intake port.
According to another aspect of the present invention, a uniflow engine comprises a cylinder having a cylinder wall, an exhaust gallery having an exhaust gallery wall, at least one exhaust port extending between the cylinder wall and the exhaust gallery wall, an exhaust channel extending from the exhaust gallery, and an exhaust valve configured to open and close the exhaust channel.
According to another aspect of the present invention, a uniflow engine comprises a cylinder having a cylinder wall, a volume exterior to the cylinder, at least one channel extending between the cylinder wall and the volume, and a valve outside of the cylinder configured to open and close flow communication between the cylinder and the volume through the channel.